1. Field of the Invention
The present invention relates to a high-strength, high-ductility cast aluminum alloy, which enables a near-net shape product to be produced through an improvement in the structure of a cast aluminum alloy, particularly through the use of specific constituents and the control of a cooling rate, and a process for producing the same.
2. Prior Art
In the case of a rapidly solidified Al alloy, the mechanical properties thereof are greatly influenced by grain shape and size. In recent years, this has led to development with attention to the cooling rate. In this case, the important properties required of Al alloys, as a structural material, are strength and ductility. These properties are, however, generally contradictory, and it has been regarded as difficult to simultaneously attain high levels of both properties.
Specifically, in the rapid solidification process, strengthening by taking advantage of precipitates of crystals is effective for increasing the strength. This, however, generally results in remarkably lowered ductility. Representative high-strength Al alloys include, for example, an alloy prepared by powder metallurgy as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1-275732. The properties of this alloy have a tendency although the strength is increased, to lower the ductility.
For the high-strength Al alloy prepared by powder metallurgy, the elongation is usually not more than several percent, and the elongation of an Al alloy, having a high Si content, prepared by powder metallurgy is 1 to 2% at the highest.
Further, for powder metallurgy, the cost for the preparation of a powder is high, in addition, the steps of bulk production, forming and the like, are necessary for commercialization, which naturally results in an increased cost.
On the other hand, an elongative material has the best-balanced properties in respect to strength and ductility. In recent years, however, no significant improvement in the properties of this material has yet been attained. In order to develop superior properties, thermomechanical treatment and other processes should be made, which are likely to increase the cost of production.
For this reason, an enhancement of the strength and ductility of a low-cost cast material to the level of those of the elongative materials is most desirable. However, the cast material, which seems to be the lowest-cost material, suffers from a problem in that the strength is much lower than that of the materials prepared by the rapid solidification process and the powder metallurgy process for the following reasons.
At the outset, in the case of the most common and effective precipitation (dispersion) strengthening, in order to provide strength, a larger amount of a strengthening phase of crystal or precipitate should be produced homogeneously and finely. However, the strengthening phase is fragile, and, in addition, the interface of the strengthening phase and the Al matrix is likely to fracture, resulting in lowered ductility. For this reason, strength should be sacrificed in order to ensure the desired ductility.
The sole method that seems to enable both the strength and ductility to be improved is strengthening by refining the structure. In order to attain a distinguishable improvement in the properties, the refinement should be significant. This requires a very high cooling rate. Eventually, the above method should rely on the powder metallurgy process, which, as described above, results in a very high production cost.